Relationship between sleep and obesity among U.S. and South Korean college students

A 40-item paper questionnaire in English was adapted from the National Health and Nutrition Examination Survey (NHANES), the American College Health Association-National College Health Assessment (ACHA-NCHA), and the Center for Epidemiologic Studies Depression Scale (CES-D). NHANES collects the nation’s primary source of health-related data [35, 36], and ACHA-NCHA [37] and CES-D [38] have been determined to be of high reliability and high validity. The original 40-item English questionnaire was translated into Korean by two bilingual health professors at two universities in Korea, using translation steps to ensure that the translated questionnaire is valid and reliable [39]. A back translation was performed by two bilingual health professors in the U.S. Then, the jury of bilingual experts in the U.S. and Korea reviewed the original and translated questionnaires and edited the translated version to remove discrepancies. The translated questionnaire was pilot tested with ten undergraduates at participating universities in Korea to evaluate the questionnaire wording and respondent’s understanding of the questions. No wording problems were found, and this pilot sample was not included in the study sample. The current study was approved by the Institutional Review Board at five participating universities in the U.S. and Korea.

Study participants were undergraduates aged 18 to 25 years at a predominantly non-Hispanic white university, a historically black university, and three universities in Korea utilizing data from academic years 2016–2017 and 2017–2018. Convenience sampling was used for this study. A total of 2232 undergraduates enrolled in health courses (e.g., Foundations in Physical Education) were invited to participate in the 40-item paper survey by an email invitation that was sent by the five health departments at the five participating universities. Lead investigators at each university provided students with an overview of the study during class sessions. Inclusion criteria were being blacks, whites, Koreans, and aged 18 to 25 years. An exclusion criterion included pregnant students. A total of 1605 students participated (72% response rate), and 27 questionnaires with 10% or more missing responses were excluded based on NHANES analytic guidelines [40]. The final sample size was reduced to 1578 participants (60.3% Koreans) on five campuses.

Race/nationality was self-reported and included non-Hispanic whites, non-Hispanic blacks, and non-Hispanic Asians. Annual family income was measured by an open-ended question “What is the total income received last year by you and your family members before taxes? Family is individuals and groups of individuals who are related by birth, marriage or adoption.” Using a Detecto scale with height bar (Detecto Scale Company, Webb City, MO), weight and height of participants in the U.S. and Korea were measured while wearing light clothing without shoes. Detecto is the largest medical scale manufacturer in the world [41] and is used as height and weight measurements in obesity studies [42, 43]. Weight and height were measured twice, and the mean of each measurement was recorded by lead investigators at each university. BMI was calculated as kg/m² using participants’ measured height in centimeters and weight in kilograms. Standard BMI cut points (underweight = BMI < 18.5; normal = 18.5 ≤ BMI < 25.0; overweight = 25.0 ≤ BMI < 30.0; and obese = BMI ≥ 30.0) [44] and Asian BMI cut points (underweight = BMI < 18.5; normal = 18.5 ≤ BMI < 23.0; overweight = 23.0 ≤ BMI < 25.0; and obese = BMI ≥ 25.0) [45, 46] were used for U.S. students and Korean students, respectively.

General health was assessed with the question, “How would you describe your general health?” with response options of excellent, very good, good, fair, and poor. Participants answered dichotomous questions regarding sleep problems: (1) “Have you ever told a doctor or other health professional that you have trouble sleeping?” and (2) “Have you ever been told by a doctor or other health professional that you have a sleep disorder?” Sleep duration was measured by an open-ended question “How much sleep do you usually get at night on weekdays or workdays?” Responses to this question were grouped into short sleep (< 7 h/night), normal sleep (7–9 h/night), and long sleep (> 9 h/night) [47]. Sleep quality was assessed with the question, “On how many of the past 7 days did you get enough sleep so that you felt rested when you woke up in the morning?” Response options ranged from 0 days to 7 days.

Depression was measured by the CES-D scale, a 20-item self-report measure with response options ranging from 0 to 3 for each item (0 = rarely or none of the time, 1 = some or little of the time, 2 = moderately or much of the time, 3 = most or almost all the time). CES-D scores of 16 or higher is considered depressed.

Based on the Centers for Disease Control and Prevention glossary [48], current smoking (i.e., ever smoked 100 cigarettes in entire life and reported current cigarette smoking every day or some days) was measured by (1) “Have you smoked 100 cigarettes in your entire life?” and (2) “Do you now smoke cigarettes every day, some days, or not at all?” Alcoho use was assessed by an open-ended question “Within the last 30 days, on how many days did you use alcohol (beer, wine, liquor)?” Daily fruits and vegetables intake was measured with an open-ended question “How many servings of fruits and vegetables do you usually have per day (1 serving = 1 medium piece of fruit; 1/2 cup fresh, frozen, or canned fruits/vegetables; 3/4 cup fruit/vegetable juice; 1 cup salad greens; or 1/4 cup dried fruit)?”

Weekly moderate-intensity exercise for ≥10 min and weekly vigorous-intensity exercise for ≥10 min were assessed by open-ended questions (1) “In a typical week, on how many days do you do moderate-intensity sports, fitness or recreational activities? Moderate-intensity sports, fitness or recreational activities cause small increases in breathing or heart rate such as brisk walking, bicycling, swimming, or golf for at least 10 minutes continuously.” and (2) “In a typical week, on how many days do you do vigorous-intensity sports, fitness or recreational activities? Vigorous-intensity activity causes large increases in breathing or heart rate like running or basketball for at least 10 minutes continuously.”

Chi-square tests were performed to identify significant differences in demographics and health behaviors among the three racial/national groups (whites, blacks, and Koreans). Data were analyzed separately for each gender and racial/national group. Racial/national differences in sleep quality were examined by using ANOVA followed by the Scheffe post hoc test. Multinomial logistic regression models were used to compute adjusted odds ratios (AORs) and investigate the association between sleep duration and three independent variables (race/nationality, gender, and BMI) with adjustment for potential confounding factors (age, annual family income, days of vigorous-intensity activity, days of moderate-intensity activity, current smoking status, days of alcohol use, having trouble sleeping, sleep disorder, and depression) based on prior research [49‐51]. Whites were used as the reference group for all multinomial regression models. The prevalence of obesity by sleep duration was illustrated separately for race/nationality in graphical plots. Poisson regression models were used to estimate adjusted incidence rate ratio and examine the relationship between sleep quality (getting enough sleep to feel rested in the morning in the past 7 days) and three independent variables (race/nationality, gender, and BMI) with adjustment for confounding variables (age, current smoking status, days of alcohol use, and depression) based on previous studies [18, 49, 52]. All analyses were conducted with STATA version 13 (STATA Press, College Station, TX).

This study has several limitations. Since this study was a cross-sectional study, cause-and-effect associations cannot be determined. Secondly, as other observational studies, there might be unmeasured confounding factors which affect the study results. For example, influences of common behavioral factors for delayed sleep onset among young people, including caffeine intake and use of electronics late at night [67], were not considered. Thirdly, although objectively measured weight status was used, self-reported sleep duration and quality were used, so this might have affected the study results. Fourthly, when interpreting the data presented in this paper, cautions should be considered because the reliability and validity of NHANES questions used in this study have not been determined. Lastly, this study used a convenience sample of college students which is likely to be biased and have a generalizability issue.